1. Field of the Invention
The invention relates to a coherent optical heterodyne transmission system comprising at least a first and a second station interconnected by means of a glass fibre, each station comprising an electrooptical converter, an optoelectrical converter and optical coupling means coupling the optoelectrical converter to the glass fibre and which optical coupling means likewise feed a first portion of the light generated by the electrooptical converter to the optoelectrical converter and a second portion of the light generated by the electrooptical converter to the glass fibre; each station comprising modulation means for modulating with an electric modulation signal the light fed to the glass fibre.
The invention likewise relates to a station to be used in a transmission system of this type.
2. Related Art
A transmission system according to the opening paragraph is disclosed in European patent application No. 354,567.
To enable transport of a baseband signal through a glass fibre, in coherent optical transmission systems the baseband signal can amplitude modulate the light signal coming from an electrooptical converter, phase modulate it or frequency modulate it before feeding the light signal to the glass fibre.
To enable demodulation of the light signals at a station with the aid of commercially available electronic components it is necessary to convert the light signal that has a very high frequency (e.g. 10.sup.14 Hz) to a light signal having a much lower intermediate frequency of e.g. 10.sup.9 Hz. For this purpose the received signal is mixed in the receiver with the aid of an optoelectrical converter with a local light signal generated by an electrooptical converter. This establishes an electrical intermediate frequency signal having a frequency equal to the differential frequency between the frequency of the received light signal and the frequency of the local light signal.
Without further measures being taken each station would need 2 electrooptical converters for enabling full-duplex transmission, that is, one in each station for generating the light signal to be transmitted and one in each station for generating the local light signal.
In the known transmission system each station comprises no more than a single electrooptical converter for generating both the light signal to be transmitted and the local light signal. A combined light signal is obtained by combining in each station by means of optical coupling means part of the light generated by the optoelectrical converter with the received light signal. By feeding this combined light signal to an optoelectrical converter the intermediate-frequency signal is obtained. In this respect it is necessary that the absolute value of the frequency difference between the light signals generated by the two electrooptical converters be equal to the intermediate frequency.
In the known receiver the second portion of the light generated by the optoelectrical converter fed to the glass fibre is modulated by means of an optical modulator included in the optical coupling means. Such a modulator is expensive and causes considerable attenuation of the light sent through it.